GRE tunnel原理：

    一、穿马甲，走出去

       GRE的原理是将3层报文封装到IP报文里，送到tunnel对端后在解开的技术。你可以把tunnel想象成一个DDN专线，tunnel口上配置的ip地址就相当于连接DDN专线的串口的IP地址。这个地址一般是内部的IP，Internet上是不认的（假设tunnel通过Internet来建）。而tunnel source和tunnel destination地址是Internet上可以路由的IP地址，用于建立tunnel，并不是tunnel口的IP。

    二、tunnel使用分类：

    1.gre tunnel

    2、IPsec over GRE 和GRE over IPsec

 三、实例

    四、报错

1.

R2#

*Mar  1 00:29:25.787: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up

R2#

*Mar  1 00:29:35.987: %OSPF-5-ADJCHG: Process 100, Nbr 4.4.4.4 on Tunnel0 from LOADING to FULL, Loading Done

R2#

*Mar  1 00:29:44.787: %TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing

*Mar  1 00:29:45.787: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to down

*Mar  1 00:29:45.803: %OSPF-5-ADJCHG: Process 100, Nbr 4.4.4.4 on Tunnel0 from FULL to DOWN, Neighbor Down: Interface down or detached

R2#

2. FPC 1 has unreachable destination


3. Tunnel0 temporarily disabled due to recursive routing

Nbr 4.4.4.4 on Tunnel0 from FULL to DOWN,

Neighbor Down: Interface down or detached


未完，待续。。。。。。